This invention relates generally to devices for efficiently transmitting light and more particularly relates to skylights for transmitting light from the sun through a roof to a room below the roof for assisting in illuminating the room with natural sunlight and for doing so in a manner that (1) maximizes the capture efficiency, which is the proportion of the light incident upon the skylight that is transmitted into the room, (2) transmits the sunlight into the room as pleasingly diffuse light and (3) protects the inhabitants of the room from UV light.
For centuries, various kinds of skylights have been recognized as desirable features of inhabited buildings. Before the existence of modern lighting, their use was principally for the utilitarian purpose of enhancing visibility within a building interior. Today, even with modern lighting, skylights not only reduce the need for artificial light and the energy they consume but also they provide the better visibility that results from bright, broad spectrum sunlight. Skylights also bring psychologically beneficial warmth into the environment as a result of the presence of natural sunlight.
The types of skylights that are currently available range from a relatively large simple skylight, that is essentially a window constructed through a roof, to a small tubular skylight or light tunnel that is essentially a tube lined with a reflective material intended to channel the sun's rays down into a room. Unfortunately, skylights also have some inherent, undesirable characteristics that require that choices and compromises be made between the desirable and the undesirable characteristics. For example, the larger a designer makes the cross-sectional area of the sunlight transmitting path into the room, the more sunlight that is captured and transmitted into the room but also the larger becomes the heat loss in winter and heat gain in summer. Similarly, the larger the skylight, the more difficult it becomes to provide sufficient roof support for the skylight and avoid water and air leaks. The tubular skylights provide an alternative with a considerably smaller footprint area to minimize those problems but, because of the relatively small area of their upper opening, their light capture is limited. Consequently, it can be appreciated that any improvement to a skylight that increases the sunlight transmitted into the room without increasing the area of the opening or cross-sectional area of the light transmission path would improve the desirable characteristics without degrading the skylight by increasing the undesirable characteristics.
One characteristic of skylights that can benefit from improvement is the sunlight capture efficiency for a low angle sun. Preferably, that capture efficiency would be improved without requiring any moving parts, which add considerable cost, and without enlarging the area of the skylight. Capture efficiency is the ratio of the light that is transmitted through the skylight and out of the lower open end of the light passage to the light incident upon the upper open end of the light passage. The quantity of incoming light and exiting light may be expressed in terms of radiant energy or luminous energy and their ratio multiplied by 100 to be expressed in percentage.
The angle of the sun is known as the sun's altitude which is the angle from the horizon to a line extending from a point on earth to the center of the sun. For any sun altitude that is greater than 0° and less than 90°, a portion of the sunlight is incident upon surfaces that form a boundary around the light transmission passage through the skylight. These boundary surfaces may be painted surfaces of surrounding frames that are common on conventional skylights or they may be reflective, including specularly reflective, surfaces that have been used for light tunnels. Because these boundary surfaces have a finite height, the sun must have an altitude above an angle, defined herein as an acceptance altitude, in order for some of the sun's rays to pass directly through the light transmission passage of the skylight without being incident upon a surface that bounds the light passage. Consequently, for any sun altitude greater than the acceptance altitude and less than 90°, a portion of the sunlight is incident upon at least one boundary surface and a portion is transmitted through the skylight without being incident upon a boundary surface of the light transmission passage. Furthermore, as the sun's altitude becomes less, the ratio of sunlight incident upon the boundary surfaces to the sunlight transmitted directly through the light transmission passage increases. For a sun altitude that is less than the acceptance altitude, all sunlight that is incident upon the upper end of the light transmission passage is incident only upon one or more boundary surfaces of the light transmission passage; that is, no sunlight is transmitted directly through the light transmission passage without reflection.
The principal purpose and feature of the present invention is to increase the sunlight capture efficiency for skylights of several types by increasing the quantity of light that exits from the skylight into the room after being incident upon the boundary surfaces of the light passage through the skylight.
Additionally, it is a purpose and feature of the present invention to particularly increase the quantity of light that exits from the skylight into the room after being incident upon the boundary surfaces from a low angle, small altitude sun, including especially from a sun that is at or below the acceptance altitude and most especially from a sun altitude that is only a few degrees above the horizon.
A further purpose and feature of the present invention is provide a skylight for which the sunlight, that is reflected from a reflecting boundary surface of the light transmission passage, is not collimated or focused but rather is highly scattered and diffused so that it does not create glare and hot spots that are unpleasant for inhabitants in a room below the skylight.
It is also an object of the present invention to provide a skylight that is relatively inexpensive and light weight and yet has structural rigidity, is easily installed, provides a high thermal insulation barrier and can provide protection against UV radiation.